This invention relates to the method of preparation and composition of an adhesive for bonding cellulose fiber sheets, especially corrugated paperboard, and more specifically to a novel carrier composition for adhesives based on amylaceous materials.
Corrugated paperboard consists of sheets of flat and corrugated paper, bonded together with adhesive. It is commonly made by (1) passing a sheet of paper, which is referred to as the medium, between fluted rolls, usually heated, to form corrugations; (2) ) applying an adhesive to the tips of the corrugations, known in the art as flutes, on one side of the medium; (3) bringing a flat sheet, which is referred to as the liner, in contact with the adhesive-coated flutes, and (4) bonding the two sheets by the application of heat and pressure. The end product, a single sheet of corrugated medium attached to a single liner, is known as single-face corrugated paperboard. Double-face board may be made by delivering single-face board to a machine known as a double-backer where a second liner may be applied to the exposed flutes of the corrugated single-face in a similar manner.
In the interest of production efficiency, it is desirable to operate the corrugator at the fastest possible rate, usually from about 300 to 700 ft/min depending primarily on board weight. Because the production rate is limited by the speed of formation of the adhesive bond, corrugating adhesives must be capable of rapid increases in viscosity so that the bond may be made quickly. Starch paste is the preferred adhesive in the industry, although adhesives based on other inexpensive materials, such as flour, are also used. Although we will primarily discuss starch pastes, it should be understood that our invention is suitable for use with adhesives based on other amylaceous materials as well.
The adhesive paste is generally formulated in two portions. The primary mix, or carrier, is an extremely viscous solution of gelled (dissolved) starch. The secondary mix contains raw, i.e., ungelled, starch plus additives to expedite the bond formation and to give desired special properties to the paste. When the two portions of the adhesive are combined, the carrier holds the raw starch particles in suspension and imparts sufficient viscosity to the mixture to permit its application by conventional techniques. The adhesive bond is established by heating the paste in situ so that gellation occurs when it is sandwiched between the flute tips and the liner. At the gel temperature, the raw starch component dissolves and absorbs water, causing a rapid increase in the viscosity of the adhesive. Thus, the carrier starch and the raw starch act as co-binding agents in joining the medium to the liner.
Ordinary, or domestic, starch pastes do not produce water-resistant bonds. In order to attain water-resistance, modifiers such as thermosetting resins are added to the finished starch pastes. Although acid cured urea-formaldehyde, melamine-formaldehyde, melamine-urea-formaldehyde resins provide the best water resistance, they are not commonly used because they require higher temperatures to gel. In starch paste technology, sodium hydroxide, or a similar alkaline material is used to reduce the gel temperatures of the pastes to about 140.degree.-155.degree. F. so that they will set rapidly during bond formation. When acidic conditions are employed, the gel temperatures of the pastes are about 155.degree.-165.degree. F. These higher gel temperatures necessitate sharp reductions in the operating speeds of the corrugators, and are therefore not economical. Since the carriers for corrugating adhesives are normally made under alkaline conditions to obtain the desired gel, acidic materials such as alum and ammonium sulfate are added when acid curing resins are to be used in the final adhesive formulations. U.S. Pat. No. 3,984,275, which is herein incorporated by reference, describes such an acid curing resin system in which polyvinyl acetate and a copolymer of vinyl acetate and ethylene are added to improve the wet bond strength of the adhesive.
Whether the adhesive is a domestic paste or one of the water-resistant variety, best results will be achieved if its viscosity, adhesive solids content, and gel temperature are within certain limits which have been well defined by the prior art. Adhesive solids should be high in order to reduce the amount of moisture which is added to the board, since this moisture must be removed during the corrugating process. However, viscosity limitations preclude very high solids since the adhesive must be capable of transfer by the application equipment. In practice, corrugating adhesives usually contain about 15-35% solids. Furthermore, adhesive performance is dependent on high molecular weight in the carrier phase, which also produces high viscosity. The viscosity of the paste before heating should be in the range of 20-80 seconds, as measured by Stein-Hall cup. It is well known that a ratio of carrier starch to raw starch of from about 1 to 3 to about 1 to 8, and a starch to water ratio of from about 1 to 2 to about 1 to 6 are required to meet the viscosity and solids content constraints. The gel temperature should be maintained as low as possible without causing premature galatinization within the adhesive application equipment. The preferred range is 140.degree.-155.degree. F. Adhesives with higher gel points require more heat to gelatinize, which can significantly reduce production rates. For example, the operation of a corrugator producing water-resistant board with an acidic cured urea resin modified starch paste was increased from 200 ft/min to 400 ft/min by converting to a paste incorporating the carrier of this invention. Gel temperature of the resin-modified starch paste was reduced from 162.degree. F. to 144.degree. F. by this modification of the adhesive.
The carrier portion of the typical adhesive is generally prepared by cooking a mixture of starch, water, and sodium hydroxide. Sodium hydroxide, which is known in the art as caustic, reduces the gel temperature of the starch in proportion to the concentration of alkali in the solution. Thus, the caustic which is added to the primary reduces the gel temperature of the carrier starch and also, to a lesser degree, that of the raw starch in the finished paste. However, excessive caustic can degrade the starch and the cellulose in the paperboard, discoloring and weakening the finished product. This degradation may also induce an unstable viscosity in the starch paste. Finally, caustic is difficult to handle safely.
Carriers containing caustic are particularly unsuitable for acid-curing resin systems. Since these pastes develop maximum water-resistance in an acidic environment, the caustic-derived carrier must be neutralized before the resins are added. When the caustic is neutralized, however, its beneficial effect on the gel temperature of the raw starch is lost, and the high gel temperatures of these pastes make reductions in corrugator speeds necessary. Furthermore, if corrugating plants make board with both alkaline and acidic pastes, the entire adhesive-mixing apparatus has to be thoroughly cleaned during changeover to prevent glue bond failure during the transition, and, in some cases, clogging of the glue system. These hindrances to productivity sparked the development of the alkaline water-resistant pastes. The alkaline pastes are not as water-resistant as the acidic pastes, but when the liner boards themselves have only a limited resistance to water, the alkaline adhesives produce adequate bonds. However, these alkaline adhesives are incompatible with the acidic resin systems which are incorporated in ridig-when-wet corrugated boxes and have become a weak link in the box structure.